"In zero gravity, the water pools in a big glob so it doesn't necessarily go down into the suit and there's not a lot of absorptive material in the helmet," lead spacewalk officer Karina Eversley said.

When asked by a reporter if Mr Parmitano could have drowned, Ms Eversley said: "He certainly had that risk today and that's why we took it so seriously".

Expert surprised by failure, says fears of 'dire emergency' were real

Marsha Smith was a space policy expert at the Congressional Research Service for three decades and is a former director of the Space Studies Board.

She says the incident could have developed into a "very dire" emergency.

"It's always dangerous when you are outside the space station," she said.

"You need some amount of time to get back to the airlock, inside the airlock, and then to re-pressurise the airlock.

"So what they were concerned about was he could actually drown inside the helmet ... they clearly expressed concern that this could have been a very dire emergency situation. And what surprises me the most is that they are surprised at this failure and that they really don’t understand it even now.

Mr Parmitano, who last week became the first Italian to make a spacewalk, did not seem to be suffering from any ill-effects from the incident.

Analysis is continuing to determine the source of the leak and assess whether the problem is peculiar to the spacesuit Mr Parmitano wore or is part of a wider issue with the suits.